1. HUVEC after incubation with LPS (1mug/ml), IL-1alpha (10 U/ml), IL-1beta (10 U/ml), IFN-gamma (1000 U/ml) for 24 hours produced more PGI_2 than the control HUVEC in response to thrombin. Monocyte-conditioned medium was prepared by the incubation of human peripheral blood monocytes in tissue culture flasks with (LPS-Mo-CM) or without LPS (Mo-CM). HUVEC after incubation with Mo-CM produced much more PGI_2 than LPS-, IL-1- or IFN-gamma-treated HUVEC.2. The effect of incubation time in making Mo-CM was examined on thrombin-stimulated PGI_2 production by HUVEC treated with the Mo-CM. The longer was the incubation time in making Mo-CM, the more was PGI_2 production by HUVEC. IL-1beta in Mo-CM was also increased with the incubation time in making Mo-CM.3. HUVEC were cultured for 24 hours with Mo-CM to which excessive-amount of anti-serum for IL-1alpha and/or TNF was added, and were then stimulated with thrombin. Consequent PGI_2 production was decreased, but still much higher than not only the control but also LPS-, IL-1- IFN-gamma-treated HUVEC. There could be produced any other cytokines from monocytes except IL-1 or TNF which stimulate PGI_2 production by HUVEC.4. When HUVEC were cultured with Mo-CM or LPS-Mo-CM, the concentration of IL-1beta in the postculture medium of HUVEC was not significantly changed, compared with Mo-CM or LPS-Mo-CM before HUVEC were cultured with. IL-1beta did not enhance the production of IL-1beta from HUVEC.